ABIDJAN, Dec 2 (Reuters) - Ivory Coast sealed all its
borders on Thursday after its election commission declared
challenger Alassane Ouattara provisional winner of a
presidential run-off, a result the top legal body rejected.

The Constitutional Council, the body that must ratify the
result that judged President Laurent Gbagbo loser of the poll,
said the commission's announcement was illegal.

Ivory Coast's military later sealed air, land and sea
borders, without giving any reasons for the move.

The media regulator said it had suspended the signal for
French broadcaster Canal Plus Horizon. Satellite channel
France24 and Radio France International FM were also off air.

After repeated delays due to wrangling within his
organisation over the results, election commission chairman
Youssouf Bakayoko announced that Ouattara had won the Nov. 28
vote with 54.1 percent of the vote.

"The electoral commission has, in accordance with the law,
handed over to the Constitutional Council the results it has
received and validated, accompanied by the result sheets,"
Bakayoko said at a hastily-organised news conference.

Soon after the election commission statement, Paul Yao
N'dre, a staunch ally of Gbagbo's who heads the Constitutional
Council, said the poll body had missed a Wednesday deadline to
issue provisional results.

He is widely expected to rule as the president wants.

"Once it's expired, the election commission is no longer
authorised to announce results," he said on state television.
"It is the Constitutional Council that is authorised to announce
decisions on the contested results."

The council has seven days to give a final tally but N'Dre
said final results could be expected in hours.

According to the election commission margin, the council
would have to cancel nearly 400,000 votes to swing the result.

French President Nicolas Sarkozy urged the council to
"respect the will clearly expressed by the Ivorian people" and
the U.N. Security Council warned Ivory Coast that it was
prepared to take "appropriate measures", a diplomatic codeword
for sanctions, against anyone thwarting the electoral process.

Later, U.N. Secretary-General Ban Ki-moon said in a
statement he "assures the people of Ivory Coast that the (U.N.
mission) ... will undertake all possible actions, within its
mandate, to help keep the electoral process on track, to
preserve peace and security in the country."

The United States also urged all parties in the Ivory Coast
to respect the results as proclaimed by the commission.

"Credible, accredited electoral observers have characterized
the balloting as free and fair, and no party should be allowed
to obstruct further the electoral process," White House
spokesman Mike Hammer said in a statement.

The International Criminal Court issued a statement on
Thursday saying it would be monitoring acts of violence.

The vote, delayed for five years, was meant to reunite the
top cocoa grower, split in two by a 2002-2003 war, but has
instead exposed north-south divisions and sparked violence.

Bakayoko surprised reporters by walking into the
U.N.-guarded hotel in Abidjan which Ouattara has made his base
and reading out results which had been blocked despite intense
international pressure for them to be published.

Cheers erupted from Ouattara supporters gathered at the
hotel, which has been placed under U.N. guard with a handful of
armoured personnel carriers outside. Ouattara called on Gbagbo
to stick by pledges he made before the poll to respect the
results and said he planned a national unity government.

A Western diplomat told Reuters Bakayoko made the
announcement in the hotel, rather than the electoral commission,
because he feared for his personal safety.

There are widespread fears the electoral dispute will erupt
into violence between Gbagbo's and Ouattara's youth supporters
or between Ouattara's supporters and security forces.

COUNCIL DECISION EXPECTED

N'Dre said that the Constitutional Council would analyse the
results and announce a definitive winner. Earlier, a spokesman
for the council said it had the power to annul results in some
provinces and tally accordingly.

Gbagbo's party has accused the rebels still controlling the
north of the country of intimidation and of rigging the poll for
Ouattara. The rebels have denied the charge and Ouattara's party
accused Gbagbo of blocking the results of a poll he had lost.

Tensions have spiked during the uncertainty.

Security forces shot dead at least four people at a Ouattara
party office in an Abidjan suburb overnight, while members of
Gbagbo's party said they had been attacked at their residence in
the same suburb by Ouattara's militants, leaving some wounded.

An opposition leader said another 12 were shot dead by
security forces in downtown Abidjan.

Fears of unrest pushed cocoa prices up over four percent in
Thursday trade. Many Ivorian exporters have suspended business.